1. Field of the Invention
The present invention relates to an apparatus and a method for exposure, and particularly to an apparatus and a method for exposure, which are used in a photolithography process step in a manufacturing process of a semiconductor integrated circuit.
2. Description of the Related Art
In a manufacturing process of a semiconductor integrated circuit (LSI) up to now, a photolithography step for forming resist patterns used as transferred images for integrated circuit patterns on a semiconductor integrated circuit substrate (hereinafter called a xe2x80x9cwaferxe2x80x9d), and an etching step for allowing the resist patterns formed in the photolithography step to function as block or stop parts and removing unnecessary portions from an under or bed film are repeatedly performed to thereby form a three-dimensional structure of LSI on the wafer.
In the photolithography step, more specifically, a photosensitive polymer film (hereinafter called a xe2x80x9cresist filmxe2x80x9d) is applied onto the wafer and thereafter exposure light such as ultraviolet light is applied to a photomask (hereinafter called a xe2x80x9creticlexe2x80x9d) formed with integrated circuit patterns by chromium or the like having a property lightproof against the exposure light on a glass substrate, and light reflected by or transmitted through an equi-magnification or reduction optical system is focused on the surface of the wafer to form images by the equi-magnification optical system, followed by exposure, thereby transferring the resultant integrated circuit patterns onto the wafer surface. This is repeatedly performed while the wafer is being moved in an X-axis direction and a Y-axis direction to expose the whole surface of the wafer, followed by execution of alkaline development, whereby resist patterns used as transferred images for the integrated circuit patterns are formed on the wafer.
While many photolithography steps are executed till the fabrication of LSI, each of the photolithography steps needs to make alignment with a resist pattern (hereinafter called an xe2x80x9cunder or bed patternxe2x80x9d) formed in a photolithography step executed prior to the above steps with high accuracy to thereby form the resist patterns. Therefore, an exposure apparatus used in the photolithography step is provided with a mechanism for accurately detecting the positions of the wafer and reticle.
Described specifically, the wafer has an outer peripheral portion formed with a cut-away portion and a surface formed with a plurality of alignment marks for the purpose of the above alignment in general. The exposure apparatus first detects the cut-away portion provided at the outer peripheral portion and makes rough alignment in a rotational direction, followed by transfer of the wafer to a wafer stage. Next, the exposure apparatus detects the separated alignment marks at at least two points by means of microscopes to enhance position accuracy in the rotational direction and subsequently detects the alignment marks at about ten points, thereby detecting the accurate position of the wafer. On the other hand, since the reticle is not affected by severe thermal stress, film stress, etc. resulting from a manufacturing process of LSI as distinct from the wafer, position detection can be performed with satisfactory accuracy under the less number of alignment marks to be measured. In the exposure apparatus, the reticle is held by a reticle stage, and alignment marks formed respectively at one pair of opposed sides of the reticle surface, i.e., two alignment marks formed symmetrically about the optical axis of the exposure apparatus are detected, thereby detecting the accurate position of the reticle. The exposure apparatus brings the wafer and reticle into registration with high accuracy by undergoing such an alignment sequence.
Meanwhile, distortion caused by aberration of a lens exists in the exposure apparatus as a factor that inhibits high-accuracy alignment. The term distortion means such an influence that when a pattern for the reticle is image-formed on the wafer surface through an image-forming optical system, the form thereof is distorted and hence a displacement is developed in the position of each image point of a transferred image.
In order to explain the influence of the distortion specifically, a transferred image 100 for an actual reticle pattern formed on a wafer surface by image-forming, and a transferred image 102 for an ideal reticle pattern are shown in FIG. 6. Since the actual transferred image 100 is distorted in form due to the influence of the distortion as shown in FIG. 6, it dose not coincide with the ideal transferred image 102. Incidentally, FIG. 6 emphatically shows distortion to explain the influence of the distortion. While only the contour of the transferred image is shown, distortion actually occurs even thereinside.
While the lens per se is designed so as not to cause such a displacement in position as a matter of course, distortion occurs due to a process error produced upon lens manufacture, an error produced upon mounting thereof to a main body of the exposure apparatus, etc. Thus, a tendency to distortion and the extent (hereinafter called xe2x80x9cform of distortionxe2x80x9d) thereof vary every exposure apparatuses. If a photolithography process is performed under the same exposure apparatus, then substantially the same distortion forms are reached every times, thus causing no large problem in terms of the alignment of the under pattern with the resist pattern formed thereon.
However, many photolithography steps executed in the LSI manufacturing process do not necessarily use the same exposure apparatus every time from the viewpoint of productivity. When exposure apparatuses different in distortion form are utilized in combination, a large position displacement occurs between the under pattern and the resist pattern formed thereon, thus causing problems such as degradation in device characteristic of LSI, yield degradation, etc. When, for example, an exposure apparatus for forming such a transferred image 50 as shown in FIG. 7(A) on a wafer and an exposure apparatus for forming such a transferred image 52 on a wafer as shown in FIG. 7(B) are utilized in combination, position displacements occur between the transferred images formed on the wafer by the respective exposure apparatuses as shown in FIG. 7(C). Incidentally, ideal transferred images are shown by dotted lines in FIGS. 7(A) and 7(B).
The present invention has been made to solve the above problems. It is an object of the present invention to provide an apparatus, which are capable of reducing a displacement in position between an under pattern and a resist pattern due to distortion.
In order to achieve the above object, there is provided an exposure apparatus, comprising reticle support means for supporting a reticle on a predetermined position, wafer support means for supporting a wafer on a predetermined position, a light source for applying exposure light to the reticle, and an image-forming optical system for focusing the light of reflected by or transmitted through the reticle, of the exposure light irradiated from the light source onto the surface of the wafer supported by the wafer support means to thereby form each image, wherein the reticle supported by the reticle support means and the wafer supported by the wafer support means, and the image-forming optical system are rotatably provided relatively about an optical axis of the image-forming optical system.